1. Field of the Invention
The invention pertains generally to the linearization of nonlinear optical transmission systems by the generation of a compensation signal and is more particularly directed to method and apparatus for compensating the nonlinearities of an optical transmission system comprising at least one of a semiconductor laser, an optical amplifier, and an optical fiber communications link by the generation of a predistortion signal.
2. Description of the Prior Art
Optical communications systems which include a semiconductor laser, an optical fiber communication link, and an optical receiver are well known in the art. These communications systems are adapted to carry a wide range of information including voice, video, and data.
The typical optical communications system includes a laser transmitter which converts an RF information signal from an electrical signal to an optical signal. The optical signal is then carried over an optical fiber communications link where it is converted back to an electrical signal by a photodetector of an optical receiver. The transmission scheme may be analog or digital and the modulation scheme amplitude, phase, or frequency, or any combination of the above.
One of the most advantageous optical communications systems from the viewpoint of simplicity and bandwidth considerations is an analog scheme where the optical intensity of a semiconductor laser is amplitude modulated. The optical transmission system, including the semiconductor laser, optionally an optical amplifier, and optical fiber communications link, is required to transduce the electrical information signal linearly into an optical signal and to transmit the optical signal linearly over the optical fiber communication link. In general, the distortions caused by the transmission system cause the system to operate in less than the optimum manner. Increasingly, this type of optical communications system is playing an important role in the delivery of high quality signals in all types of CATV architectures.
Distortion in optical transmission systems can originate from several different sources. One of the primary sources is the electrical to optical transducer, a laser diode in most systems. Another contributor is the optical communications link and, more recently, any optical amplifier in the optical link. Some of these sources produce similar distortion signals which may even cancel others, but usually each distortion has its own unique characteristics and should be compensated for independently.
A laser diode generally exhibits at least three different types of identifiable distortion. The first type is generally at that caused by the non-linearities of its LI (light intensity as a function of electrical current) transfer function. In general, the non-linearity of the LI transfer function is similar to the function y=x.sup.2. The non-linearity may be superlinear (L increases at increasing rates for increases in I), or it may be sublinear (L increases at decreasing rates for increases in I). The next type of laser diode distortion is mainly second order and higher intermodulation products. This type of distortion can be corrected by generating a distortion signal which is generally similar (sum and difference beats of the carrier frequencies) with a similar amplitude but opposite phase. Another characteristic distortion of the laser diode is where the amplitude and phase of the modulating signal is distorted as a function of frequency changes. To correct for such a distortion, a compensation signal which varies in amplitude and phase as a function of frequency is advantageous.
The distortion generated by an optical link is generally caused by phase and amplitude dispersion. Generally, phase dispersion causes the different modulating frequencies of an optical signal to be phase shifted different amounts. Dispersion is also a function of the length of the optical link and generates different amplitudes for the modulating frequencies. Present optical communications systems use a 1330 nm. optical wavelength signal to minimize phase dispersion. However, newer systems that operate at a 1550 nm. optical wavelength are capable of minimizing amplitude signal losses but with effect of increasing phase dispersion. It would be advantageous to compensate for the increased phase dispersion of the 1550 nm. system while maintaining its lower optical loss. Optical amplifiers, particularly Erbium doped fiber amplifiers (EDFA), allow greater lengths of the optical fibers to be used without the loss of optical signal amplitude, but produce their own distortion in the form of a amplitude versus frequency characteristic.
A compensation signal to compensate for distortion in the optical link and any optical amplifier should have an amplitude versus frequency characteristic and the ability to apply it at different phases of the signal. Advantageously, the amplitude versus frequency characteristic should be independent of any amplitude versus frequency compensation signal for the laser diode as the distortion characteristic which is generated by each respective element is considerably different.